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February 2007

Lazurite is the chief mineral in the treasured gemstone known as lapis lazuli, often referred to simply as "lapis." The intense blue background of this gemstone together with its golden specs of pyrite (called "Fool's Gold" because of its resemblance to actual gold) give it a radiant appearance that some compare to the beauty of a starry night sky.

The First Synthetic Fiber

Photograph of Lapis Lazuli
From Wikimedia Commons

The chemical formula for lazurite is complicated:
(Na, Ca)8(AlSiO4)6(SO4,S,Cl)2. Most minerals, which tend to incorporate many chemical elements, have long formulas. Geochemists--geologists who study Earth chemistry--tell us that lazurite forms in places below the Earth's surface that heat up as a result of being close to magma. Magma is very hot, molten rock found under the Earth's surface. (When magma reaches the Earth's surface through a volcano or fissure, it is called lava.)

Most gemstones are minerals with distinct chemical formulas. Lapis is more properly described as a rock because it is composed of several minerals, including the golden pyrite (FeS2), white or clear calcite (CaCO3) and the blue, chlorine-containing lazurite.

Lazurite: Historical Uses

Much of the world's lapis comes from a deposit in the mountains of northern Afghanistan that has been mined for about 6,000 years. The Italian traveler and explorer
Marco Polo described this deposit in a book about his journey to Asia during the thirteenth century. Through the
millennia, lapis has been crafted into jewelry and other objects of art. Archaeologists working in Egypt report articles of lapis in the tombs of pharaohs, the kings of ancient Egypt who were buried with riches and supplies for their needs in the after-life.

Powdered lazurite is
needed to make ultramarine
From Wikimedia Commons

The lazurite found in lapis was used for ages to make the vibrant blue pigment known as ultramarine. To produce ultramarine, lazurite was carefully separated out of lapis and ground to a powder that would then be mixed with a binding agent. Because lapis was so precious, ultramarine was very expensive and its use was reserved by prominent artists for very special paintings. It wasn't until the early nineteenth century that a synthetic blue pigment of similar intensity was invented.

Recent research by professors at New York University and the Pratt Institute indicates ultramarine's color is produced by the presence of sulfur atoms in the chemical structure (see "S" for sulfur in the chemical formula). The researchers found that under very humid conditions ultramarine's color fades as sulfur is released from the structure, producing a loss of color intensity informally known as "ultramarine sickness." The researchers hope that their work will help assist art preservation efforts in the future.

Geochemistry of Lazurite Formation

Compared to the carefully controlled experiments of chemistry lab class, Earth chemistry is very complex. Geochemists have the enormous challenge of puzzling out how rock and mineral matter form and change chemically. Many geochemical reactions, including the one for the formation of lazurite, occur within the Earth where temperatures and pressures are much higher than at the Earth's surface.

Lazurite can be thought of as a product of a chemical reaction between the mineral nepheline, similar chemically to the common mineral feldspar, and sodium chloride,
NaCl, the mineral halite. As a common, widespread chemical element, chlorine plays a role in numerous geochemical reactions and appears in many mineral formulas. Chlorine in lazurite is supplied by sodium chloride. The "driving force" for this reaction is the heat supplied by nearby magma.

All photographs courtesy of Wikimedia Commons

Follow-up Activities:

What are some other examples of natural chemical reactions that occur in and on the Earth besides the formation of minerals?

What are the characteristics of a gemstone?

How is it possible to distinguish pyrite from gold?

For a list of previous "Chlorine Compound of the Month" features, click
here.